National Pipe Thread Tapered Thread (NPT) is a U.S. standard for tapered threads used on threaded pipes and fittings. The taper rate for all NPT threads is 1â„16 (Â¾ inch per foot) measured by the change of diameter (of the pipe thread) over distance. The taper divided by a center line yields an angle 1Â° 47' 24 or 1.7899Â° as measured from the center axis. Commonly-used sizes are â…›, Â¼, â…œ, Â½, Â¾, 1, 1Â¼, 1Â½, and 2 inch, appearing on pipes and fittings by most U.S. suppliers. Smaller sizes than those listed are occasionally used for compressed air. Larger sizes are used less frequently because other methods of joining are more practical at 3 inches and above in most applications. It is defined by ANSI/ASME standard B1.